1. Field of the Invention
The present invention relates to a tire shape measuring system for capturing an image of a line of light (an image of a light-section line) emitted to the surface of a relatively rotating tire and detecting the surface shape of the tire by detecting the shape using the captured image and a light-section method.
2. Description of the Related Art
In general, in order to control the quality of products in factories, the surface shape of a product (the distribution of the height of the surface of a product) needs to be contactlessly measured at high speed.
Tires are manufactured by laminating a variety of materials, such as rubber, chemical fabrics, and steel cords. If the laminated structure has a non-uniform portion and the tire is filled with air, a protrusion called a “bulge” or a recess portion called a “dent” or a “depression” is generated in a portion having a relatively weak resistance to pressure. After being inspected, such defective tires having a bulge or a dent should not be shipped for a safety reason or an appearance reason.
In existing inspection methods for detecting a defective tire shape, the surface heights of a plurality of points of a tire are measured using a contact or non-contact point-measuring sensor while the tire is being rotated by a rotator. Subsequently, the surface shape of the tire is measured using the distribution of the surface heights. However, in the inspection method for detecting a defective tire shape on the basis of measurement of the tire shape using a point-measuring sensor, the number of the sensors arranged and the time available for inspection are limited. Accordingly, the entire surface shape of the tire to be measured cannot be measured at one time, and therefore, some defective tires may pass through the inspection process.
In contrast, Japanese Unexamined Patent Application Publication No. 11-138654 describes a technology in which slit light (a line of light) is emitted to the surface of a rotating tire, and the image of the slit light is captured. The surface shape of the tire is measured by detecting the shape using a light-section method using the captured image. This technology allows the entire shape of the surface (the sidewall surface or a tread surface to be totally (continuously) measured. Thus, failure to detect a tire having a defective shape can be prevented.
As described in Japanese Unexamined Patent Application Publication No. 11-138654, in general, in order to detect a shape using a light-section method, a line of light is emitted to the surface to be detected (e.g., the sidewall surface of a tire) so that a light-section line (a linear portion to which the light is emitted) is formed in a direction in which the height of the line light is detected (the height direction a surface to be detected). Thereafter, diffusely reflected light of the line of light is captured by a camera disposed in a particular direction so that the linear image of the line of light (the image of the light-section line) is captured.
In general, the surface of a tire and, in particular, the sidewall surface of the tire is black and glossy. Thus, the possibility of a line of light emitted to the surface of the tire being diffusely reflected is relatively low. In addition, the entire surface of a tire and, in particular, the entire sidewall surface of the tire has a convex shape. Accordingly, in order to obtain a desired depth of field, the aperture of the camera should be sufficiently small.
Accordingly, in the measurement process of a surface shape described in Japanese Unexamined Patent Application Publication No. 11-138654, in order to obtain a clear image of the line light, the intensity (light amount) of the line light needs to be increased or the capture rate (the shutter speed) of the camera needs to be decreased so that the exposure time is increased.
However, if the intensity of the line of light is increased, the black tire that easily absorbs the light may be damaged by heat. Furthermore, if a high-power light source (typically, a laser light source) is used, a cooling unit is required. Accordingly, a measuring system is increased in size and cost. Still furthermore, the maintainability disadvantageously decreases.
In addition, in order to capture the image of a light section line in a circumferential direction of the rotating tire with sufficient spatial resolution within a limited time allowable for product inspection, the capture rate (the number of image captures per unit time) cannot be decreased to that sufficient for capturing a clear image of the line of light.
For example, the time allowable for inspecting a defective shape of a tire is 1 second per tire. In addition, in order to distinguish the image of a light section line from a letter printed on the surface of the tire using a light-section method during tire shape measurement, the image needs to be captured with a spatial resolution smaller than or equal to the line width (about 1 mm) of the letter in the circumferential direction of the rotating tire. In order to satisfy the conditions about the inspection time and the spatial resolution, 2000 frames needs to be captured per second for a tire of a passenger vehicle, and 4000 frames needs to be captured per second for a tire of a truck or a bus that is larger than that of a passenger vehicle. Recently, the speed of an image capturing process of an image sensor has been increased. For example, a high-speed image sensor, such as a complementary metal-oxide semiconductor (CMOS) sensor, can capture images as fast as 2000 to 4000 frames per second. However, if images are captured at a high rate of 4000 frames per second, it is difficult to obtain a clear image of the light line using a technology described in Japanese Unexamined Patent Application Publication No. 11-138654.
In addition, during measurement of a tire shape using a light-section method, a process for extracting an image of a light section line (the image of a line of light) from each of the captured images (a one-frame image) is needed. That is, an imaging process for detecting the position (the coordinates) of a light section line having high luminance needs to be performed on the basis of the luminance information about each of pixels of the captured image so that the surface shape (the height of the surface) of the tire is determined using the extracted image of the light section line (i.e., the coordinates of the extracted image). In general, a light section line is extracted by identifying the position (the x coordinate) of a pixel having the highest luminance in a pixel group of each of the horizontal lines of the captured image (for each of positions of the y coordinate). In the sub-processes based on the captured image (the luminance information of each of the pixels of the captured image) of the inspection process for detecting a defective shape of a tire, the extraction sub-process of a light section line makes up almost all of the processing load. In Japanese Unexamined Patent Application Publication No. 11-138654, the extraction sub-process of a light section line is performed by a microcomputer (a micro processing unit (MPU)) including a central processing unit (CPU), a read only memory (ROM), and a random access memory (RAM).
Recently, there has been a growing need to obtain the result of detection of a defective shape substantially in real time because of improved processing time of a process of capturing an image of a tire surface (e.g., 1 second) in the inspection process for detecting a defective shape of a tire.
However, because of a limitation of the processing power of a practical MPU that can be used as a component of an inspection system (a measuring system) for detecting a tire having a defective shape, it is difficult for the MPU to perform the extraction process of a light section line on the basis of a captured image within 1/2000 to 1/4000 seconds after the image (the luminance information about an image for one frame) is captured. Accordingly, in the existing inspection process for detecting a defective shaped tire, information about the image captured by an image sensor needs to be stored in a large-capacity memory (e.g., a hard disk). Even after an image capturing process is completed, the extraction process of the light section line on the basis of the captured image (the imaging process performed by the MPU) needs to be performed. As a result, the production efficiency of a tire is decreased.